Reissue Patent, Re. No. 32,395 discloses several forms of compression piston ring construction including a thin multi-sector ring having free circumferential curvature equal to the cylinder wall wherein each sector is provided with a "microspring" formed by partial radial cuttings leaving a spring element therebetween. Each of the three sectors is made slightly greater than 120.degree., so that circumferential compression upon installation will maintain radial cylinder wall contact with sufficient microspring clearance remaining at the cuttings to accommodate thermal expansion in operation. As a sealing element for clearance gaps at the microsprings, a supplemental single piece plastic ring, made of a material such as Teflon capable of operation without gap at its single cutting ends, is disclosed for installation under the metal ring sectors on the crankcase side to seal against any leakage through the microspring partial cuttings.
Such supplemental plastic sealing ring has been found satisfactory in operation where temperatures do not exceed 400.degree.; however, higher operating temperatures in the order of 500.degree. encountered in high performance engines exceed the operating temperature limits of the plastic.